The present invention relates generally to the design and manufacture of material processing apparatus and, more specifically, to consumables used in the apparatus and methods for aligning the consumables with an axis of the apparatus.
Material processing apparatus, such as plasma arc torches and lasers, are widely used in the cutting, welding, and heat treating of metallic materials. A plasma arc torch generally includes a cathode block with an electrode mounted therein, a nozzle with a central exit orifice mounted within a torch body, electrical connections, passages for cooling and arc control fluids, a swirl ring to control fluid flow patterns in the plasma chamber formed between the electrode and nozzle, and a power supply. The torch produces a plasma arc, which is a constricted ionized jet of a plasma gas with high temperature and high momentum. Gases used in the torch can be non-reactive (e.g. argon or nitrogen), or reactive (e.g. oxygen or air).
Similarly, a laser-based apparatus generally includes a nozzle into which a gas stream and laser beam are introduced. A lens focuses the laser beam which then heats the workpiece. Both the beam and the gas stream exit the nozzle through an orifice and impinge on a target area of the workpiece. The resulting heating of the workpiece, combined with any chemical reaction between the gas and workpiece material, serves to heat, liquefy or vaporize the selected area of workpiece, depending on the focal point and energy level of the beam. This action allows the operator to cut or otherwise modify the workpiece.
Certain components of material processing apparatus deteriorate over time from use. These xe2x80x9cconsumablexe2x80x9d components include, in the case of a plasma arc torch, the electrode, swirl ring, nozzle, and shield. Ideally, these components are easily replaceable in the field. Nevertheless, the alignment of these components within the torch is critical to ensure the reasonable consumable life, as well as accuracy and repeatability of plasma arc location, which is important in automated plasma arc cutting systems.
In a plasma arc torch, the location and angularity of the arc is determined by the relative location of the electrode and nozzle or, more specifically, the location of an insert disposed in a tip of the electrode relative to a centerline of the nozzle orifice. Since the plasma gas flowing through the orifice tends to center the arc in the orifice, it is desirable that the insert is concentrically aligned with the orifice, as any misalignment skews the arc relative to the centerline datum of the torch. As used herein, the term xe2x80x9caxially concentricxe2x80x9d and variants thereof mean that the centerlines of two or more components are substantially collinear. Depending on the direction of cut, any misalignment can result in the production of parts with improper dimensions and non-normal edges. Asymmetric wear of the nozzle orifice also typically results, requiring premature replacement of the nozzle.
Tolerances associated with conventional methods of mounting the electrode and nozzle render systems employing such torches incapable of producing highly uniform, close tolerance parts due to the errors inherent in positioning the electrode relative to the nozzle. One method of mounting the electrode and nozzle employs close tolerance sliding fits. For example, a cathode block having a bore for receiving a base of the electrode has a nominal diameter of 0.272 inches (0.691 cm) with a machining tolerance band of plus or minus 0.001 inches (0.003 cm). Accordingly, the bore can have a maximum diameter of 0.273 inches (0.693 cm) and a minimum diameter of 0.271 inches (0.688 cm). In order to ensure the electrode can be inserted reliably in the block without interference, the electrode base has a nominal diameter of 0.270 inches (0.689 cm) with a machining tolerance band of plus or minus 0.001 inches (0.003 cm). Accordingly, the electrode base can have a maximum diameter of 0.271 inches (0.688 cm) and a minimum diameter of 0.269 inches (0.683 cm). The diametral clearance between the base and bore can range between zero and 0.004 inches (0.010 cm) yielding a maximum radial displacement of the electrode relative to a centerline of the torch of 0.002 inches (0.005 cm). This maximum radial displacement is also called the worst case stacking error which results from employing a minimum allowable diameter electrode base with a maximum allowable diameter cathode block bore.
The worst case stack error of the nozzle is added to that of the electrode to determine the combined total maximum radial displacement for the nozzle and electrode in the torch. Calculation of nozzle location error is similar to that of the electrode. For example, a torch body having a bore for receiving a base of the nozzle has a nominal diameter of 0.751 inches (1.908 cm) with a machining tolerance band of plus or minus 0.001 inches (0.003 cm). Accordingly, the bore can have a maximum diameter of 0.752 inches (1.910 cm) and a minimum diameter of 0.750 inches (1.905 cm). In order to ensure the nozzle can be inserted reliably in the body without interference, the nozzle base has a nominal diameter of 0.747 inches (1.897 cm) with a machining tolerance band of plus or minus 0.002 inches (0.005 cm). The larger tolerance band is attributable to the increased difficulty of machining larger diameter parts to close tolerances reliably at reasonable cost. Accordingly, the nozzle base can have a maximum diameter of 0.749 inches (1.902 cm) and a minimum diameter of 0.745 inches (1.892 cm). The diametral clearance between the base and bore can range between 0.001 inches (0.003 cm) and 0.007 inches (0.018 cm) yielding a maximum radial displacement of the nozzle relative to a centerline of the torch of 0.0035 inches (0.0089 cm).
The combined total maximum radial displacement of the nozzle relative to the electrode is the sum of the individual maximum radial displacements or 0.0055 inches (0.0140 cm). For a torch having an axial distance between a tip of the electrode insert and an entrance to the nozzle orifice of 0.140 inches (0.3556 cm), the angularity of the arc relative to the torch centerline may be related to the angularity of the consumables relative to the torch centerline, the latter of which is calculated geometrically as about 2.25 degrees. Accordingly, if the axial distance from the tip of the insert to the workpiece surface is 0.274 inches (0.696 cm), the maximum dimensional error from the centerline of the torch projected on the workpiece to the actual entrance of a cut on the workpiece may be calculated geometrically as about 0.0108 inches (0.0274 cm). Depending on the direction of arc misalignment and the direction of the cut, the component cut from the workpiece may have cut edge angularity of 2.25 degrees and the dimensional error of the finished part may be up to twice the 0.0108 inches (0.0274 cm), or 0.0216 inches (0.0549 cm), in the case where opposite edges of the workpiece are both cut with the maximum skew. This magnitude of errors is unacceptable for reliably producing parts and features therein having total dimensional tolerance of between about plus or minus 0.005 inches (0.013 cm) and about plus or minus 0.010 inches (0.025 cm). Further, for a small nominal diameter nozzle orifice such as 0.018 inches (0.046 cm), the combined maximum radial displacement of 0.0055 inches (0.0140 cm) and angularity of 2.25 degrees result in asymmetric wear of the nozzle entailing premature replacement.
Diametral tolerances of plus or minus 0.001 inches (0.003 cm) for each of an electrode base, cathode block bore, and torch body bore and plus or minus 0.002 inches (0.005 cm) for a nozzle base are necessary to ensure the capability to replace readily the consumable components in the field. While tighter tolerances could be employed, such practices typically would entail higher manufacturing costs and likely complicate the field replacement of the consumables. Attempts to rely on O-rings for sealing the radial clearances as well as centering are ineffective since there exists substantial inherent variation in the molded cross-sectional profiles of O-rings.
Instead of using close tolerance sliding fits, the electrode and nozzle may be mounted on the cathode block and torch body, respectively, by means of screw threads. Based upon thread data tabulated in Machinery""s Handbook, 24th Edition (Industrial Press, Inc. 1992), for an electrode and cathode block pair employing a {fraction (5/16)}-20 UN thread, the worst case stack clearance based upon pitch diameter is 0.0104 inches (0.0264 cm) yielding a maximum radial displacement of the electrode centerline relative to the torch centerline of 0.0052 inches (0.0132 cm). For a nozzle and torch body employing a xc2xe-12 UN thread, the worst stack clearance based upon pitch diameter is 0.0144 inches (0.0366 cm) yielding a maximum radial displacement of the electrode centerline relative to the torch centerline of 0.0072 inches (0.0183 cm). Accordingly, the combined total maximum radial displacement is 0.0124 inches (0.0315 cm) yielding an angular error of 5.06 degrees and a dimensional error of 0.0242 inches (0.0615 cm) for a torch having similar axial dimensions as in the aforementioned example. While more precise threads could be employed, manufacturing costs would increase as well the difficulty associated with assembly and disassembly, especially since the threads are subject to surface degradation and thermal deformation in use.
Another method of providing axially concentric alignment of the electrode and nozzle involves the use of mating taper fits with the respective cathode block and torch body. While improved concentricity may be achieved, relative and absolute axial location of the electrode and nozzle suffer. In effect, tapers convert radial errors to axial errors. For example, for a nominal taper included angle of 30 degrees relative to torch centerline and a tolerance of plus or minus 30 minutes, the maximum axial displacement of an electrode relative to a cathode block is about 0.0047 inches (0.0120 cm).
Component axial accuracy is important for proper torch operation. For example, numerous elements are nested in the torch assembly, many of which are captured, such as the swirl ring disposed between the electrode and nozzle. Accordingly, it would be very difficult to ensure seating of both electrode and nozzle tapers while meeting the requisite axial stacking dimension of interdisposed components. Further, the relative distance between the electrode and the nozzle should be controlled within a narrow range. The distance therebetween should be large enough to provide for reliable pilot arc initiation, yet not so large as to exceed the breakdown voltage of the power supply in arc initiation mode. Additionally, and perhaps more importantly, the length of the transferred arc from the tip of the electrode at the insert to the workpiece should be closely controlled to achieve proper control of the power and proper processing of the workpiece. Changes in arc length affect arc voltage, which in turn effects other critical processing parameters in the power supply.
Another method of providing axially concentric alignment of consumables in a plasma arc torch is disclosed in U.S. Pat. No. 5,841,095 to Lu, et. al., and assigned to the assignee of this application. The disclosure of this patent is incorporated herein by reference in its entirety. Briefly, this patent discloses centering of electrodes and nozzles in plasma arc torches using radial spring elements. It has been determined, however, that at higher electrical current carrying requirements, such radial spring elements increase significantly in size and require major redesign of the cathode block, current ring, and other components of the torch tip or output structure.
Accordingly, there exists a need to improve upon the current state of the art by providing low-cost, readily-manufacturable, and easily-replaceable consumables in a streamlined output structure of a material processing apparatus, where the alignment and concentricity of consumable components in the output structure can be closely controlled. The capability to retrofit existing apparatus with minimal modification is also highly desirable.
In one embodiment, the invention provides an output structure for material processing apparatus that facilitates field replacement of consumable components while maintaining critical alignments. By ensuring the proper alignment of the consumables, the accuracy of apparatus operation and the lifetimes of the consumables are improved.
The output structure includes a contoured alignment surface and a consumable component that also has a contoured surface. When installed in the apparatus, the contoured surface of the consumable component mates with the contoured alignment surface of the output structure. This mating action serves to facilitate alignment of the consumable component with an axis of the output structure.
Examples of typical material processing apparatus include plasma arc torches and lasers. In some embodiments, the consumable component is an electrode, a swirl ring, a nozzle or a shield. The contoured surfaces include linear tapers and arcuate sections in any combination. For example, in an embodiment including an electrode, an outer surface of the electrode is contoured over an axial extent of less than about 0.5 inches (1.27 cm) and, in some embodiments, less than about 0.25 inches (0.635 cm). In an embodiment incorporating an electrode with a linear taper, the angle formed between the taper and the axis of the electrode can be any value less than 90 degrees. In an embodiment incorporating an electrode with a contoured surface that is an arcuate section, the arcuate section can have a fixed radius of curvature or several radii of curvature.
In one embodiment, a plasma arc torch includes a consumable swirl ring, the swirl ring having a surface contoured over an axial extent of, for example, less than about 0.5 inches (1.27 cm). The contoured surface may be linear taper surface where the angle formed between the taper and the axis of the swirl ring can be any value less than 90 degrees, for example, less than about 45 degrees. In another embodiment, the contoured surface may be an arcuate section defined by a fixed radius of curvature or several radii of curvature.
In another embodiment, a plasma arc torch includes a consumable nozzle, the nozzle having a surface contoured over an axial extent of, for example, less than about 0.5 inches (1.27 cm). The contoured surface may be linear taper surface where the angle formed between the taper and the axis of the nozzle can be any value less than 90 degrees, for example, less than about 45 degrees. In another embodiment, the contoured surface may be an arcuate section defined by a fixed radius of curvature or several radii of curvature.
In yet another embodiment, a plasma arc torch includes a consumable shield, the shield having a surface contoured over an axial extent of, for example, less than about 0.5 inches (1.27 cm). The contoured surface may be a linear taper surface where the angle formed between the taper and the axis of the shield can be any value less than 90 degrees, for example, less than about 45 degrees. In another embodiment, the contoured surface may be an arcuate section defined by a fixed radius of curvature or several radii of curvature.
To provide axial retention upon installation in the output structure, the consumable component may include a threaded surface for engaging a cooperating thread of the output structure. Alternatively, in xe2x80x9cblow forwardxe2x80x9d or xe2x80x9cblow backxe2x80x9d type plasma arc torches, such as those described in U.S. Pat. Nos. 5,994,663 and 4,791,268, respectively, the disclosures of which are incorporated herein by reference in their entirety, the electrode or nozzle can translate axially in the torch from a contact start position to a separated pilot arc position using a sliding fit in a suitably sized bore. In such an embodiment, one or more spring elements may be included to bias at least one of the components in the axial direction. Accordingly, during operation of the torch the consumable is seated in an aligned orientation and maintained at the correct axial location due to the pressure in the plasma chamber.
In another embodiment of the invention, the output structure includes a second contoured alignment surface and a second consumable component that also has a contoured surface. Similar to the embodiment discussed above, the contoured surface of the second consumable component mates with the second contoured alignment surface of the output structure. This facilitates alignment of the second consumable component with the same axis of the output structure, such that both consumables are concentrically aligned.
In some embodiments, the second consumable component can be an electrode, a swirl ring, a nozzle, or a shield. The second contoured alignment surface, as well as the contoured surface of the second consumable component, can be, by way of example, linear taper surfaces or arcuate sections.
To retain its axial position within the output assembly, the second consumable component may include a threaded surface that engages a cooperating thread on the output structure, or may include a sliding fit in a suitable sized bore as discussed above for translatable component designs.
In another embodiment of the invention, a tool is used for installing and aligning a consumable component with an axis of the output structure of a material processing apparatus. The tool typically has a body with an outer contoured mating surface for mating with a contoured surface of the output structure. Further, the body generally includes a bore with an inner drive surface. The bore is sized to receive the consumable component and the inner drive surface engages a keyed surface of the consumable component. The tool may be used to thread the consumable component onto a threaded surface of the output structure, while simultaneously providing radial support to center the electrode. In some embodiments, the consumable component may also include a deformable surface that conforms to the output structure so as to maintain alignment with the axis of the output structure when the tool is removed.
In an embodiment where two consumable components are aligned with the axis of the output structure as described above, the components are consequently also concentrically aligned with each other. This is exemplified by a nozzle which, as the second consumable component, is typically installed so as to circumscribe the previously installed consumable electrode. In this configuration, the output structure, electrode, and nozzle all share a common axis. In an alternative embodiment, a third consumable component, such as a swirl ring, is also centered and shares the common axis.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating the principles of the invention by way of example only.